4.2 pistons

CastleMGBV8 · Post by **CastleMGBV8** » Wed Oct 14, 2009 9:01 am

Dimitri,

I used 5.85" Rods and comp height of the pistons is 1.561" so had to take 40thou. of the decks to get the pistons within 40 thou. of the top of the bore.

The pistons have valve cut outs (chevy pattern) of 5cc.

The forged pistons Real steel are now offering for 5.0 stroker kit at £350 look interesting but don't know the comp height, would be worth finding out.

Kevin

paul c · Post by **paul c** » Thu Oct 15, 2009 8:11 am

cranks on its way, cost 190GBP including shipping so I am happy as a ...

Dimitri, if you are still looking for a crank seems they have a few available at turner engineering.

Wotland · Post by **Wotland** » Thu Oct 15, 2009 9:26 am

Many thanks Paul,

I contacted Frida at Tuneer Engineering and crank already ordered

.

CastleMGBV8 · Post by **CastleMGBV8** » Thu Oct 15, 2009 2:59 pm

I can confirm that with the 4.2 77mm. crank that the engine is very free revving and very nice to drive.

I have what might be called the the equivalent of stage 3 big valve heads and a Crower 50232 cam that will pull 4th gear from 1500 RPM.

Kevin

paul c · Post by **paul c** » Thu Oct 15, 2009 8:22 pm

Thanks Kevin, looking forward to getting it built, must get started on the manifold conversion while I gather the pieces for the block. I have a 3.9 cam bought from Autopost, but a little unsure whether to use it or not, Island 4x4 have 4.2 cams but again not sure of the supplier, they all use the Rover part number for reference so may not be genuine.

Wotland · Post by **Wotland** » Fri Oct 16, 2009 8:49 am

Island 4X4 cams are Britpart parts. Not really good quality and genuine LR camshaft was designed to be used in low comp 4.2 engine.

paul c · Post by **paul c** » Fri Oct 16, 2009 7:01 pm

Thanks Dimitri, I suspected they were either Britpart or a lot of the parts now are labelled Allmakes.
Do you have any idea of Autoposts supplier, I have a 3.9 cam from them I am unsure of using, I believe the 3.9 is a good usable cam, seems to work well in most engines, but is Autopost quality okay or should I try to find a similar grind, I believe this cam is made suitable for using a cam retaining plate and distributor drive gear?

Charlie_b · Post by **Charlie_b** » Fri Oct 16, 2009 7:06 pm

So, from this topic, am I right in thinking that you can't get high compression pistons for a 4.2L?

Surely there must be some uprated pistons available for a 4.2 or is the only way to skim the fire face down and pocket the standard pistons?

CastleMGBV8 · Post by **CastleMGBV8** » Fri Oct 16, 2009 7:52 pm

Charlie,

I did a big search when I was going to start my new engine build and coudn't find any other 4.2 pistons, which explains why some of the V8 tuning guys do the modded 3.9 L/comp piston dont know if that could be done with the High comp version of the 3.9 piston, and I dont particularly like the idea of weakening the top ring land if the engine is going to be fairly highly tuned.

I suspect they only machine enough off to bring the piston just below the deck heights which is good for compression and possibly a bit more squish, but that still means they will have to remove at least 20-30thou. from the outer raised ring area or possibly more.

I do have a spare set of very good 3.9 H/comp pistons and rods though.

Kevin.

paul c · Post by **paul c** » Tue Oct 27, 2009 7:34 am

Crank is here, looks good so can get collecting other bits for the build, is real steal the best place for pistons or somewhere else to look, any suggestions?

bones · Post by **bones** » Tue Oct 27, 2009 12:54 pm

i got my 3.9 pistons from real steel, the place i had them balanced said they were a very good casting and my 3.5 pistons from them survived a 125 shot of nitrous

, and they are cheap

rich

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 27, 2009 2:24 pm

Paul,

Your cheapest option using the standard rods would be the machined L/C 3.9 pistons from V8 Tuner. anything else and your into custom rods and pistons and lots of machining.

http://v8tuner.co.uk/

Kevin.

seight · Post by **seight** » Tue Oct 27, 2009 4:01 pm

My 4.2 has machined 3.9 +.020 low comp pistons. I was advised the following from Glen Towery in the states :
"Michael, you need to use a 3.9 low comp. piston & ALWAYS ck & make sure the
piston sits .010 below the block deck. Hepolite or rover O/E pistons are
best. We take .090 off the top of the piston & .125 off the bottom. Do not
use high comp. pistons, the comp. ratio will be to high in the 4.2. CHEEK
THE 4.2 BLOCK!!!! We have had a LOT of problems with this block (leaks
water to sleaves moving (sleave moving is what happened to the NEW 4.2 short
block that I put in my G.T.)
Use the steel shim gasket!!! We do not have a problem with the steel
shim!!! & we only put 55 to 60 lbs. tork on the head bolts & we reuse the
O/E head bolts. We use a molly ring with a very fine hone job on the bore,
over the chrome rings the factory uses. Hope this helps Glenn"

Oh and with my setup I'm using composite gaskets with 34cc heads. I had the cylinders cc'd at tdc first to check all my measurements. The guy that cc'd it for me was the guy that then messed up on skimming the block so maybe I should double check his measurement

However I've not run it yet and I had to pocket the pistons so I've no idea whether it will all stay together!

Mike

paul c · Post by **paul c** » Tue Oct 27, 2009 8:42 pm

Kevin, thanks, I see the standard LC pistons listed but not machined for a 4.2.

There is a cnc machinist here who has modded pistons for me before, or there is an ebay seller (DJEV8 ?) who has them listed already done.

Thanks Mike, I have a 3.9 serp block (95 interim) that I am running which has no known problems I am going to use, I intend to do a trial fit of the crank with stock rods and pistons first to do some measurements. I intend to use the composite gaskets and will fit ARP studs to heads and mains. When I got this engine it had steel gaskets and stretch bolts but the heads were loose so I fitted composites and early bolts, it is a 10 bolt block and heads.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 27, 2009 9:21 pm

Paul,
As you have the 10 bolt 28cc heads you could use tin gaskets to get an extra
.5/1 compression.

Check with Paul at V8 Tuner first as DJE V8 tend to be a bit more pricey.

Try and find out the volume of the bowls of the modified pistons then you can work out what your compression will be, the fact that you are machining the edge rim of the L?C piston will probably turn it into a H/C piston anyway.

Kevin.